1. Field of the Invention
The invention relates to the field of small microwave transponders for "wireless key" and surveillance applications, and in particular to transponders that are activated by an interrogation beam to upconvert and radiate a digitally modulated identification tone.
2. Description of the Prior Art
The need for automatic identification of objects and personnel has grown rapidly in recent years with the increased use of computerized systems for security and control tasks. Noncontact identification schemes using radio frequencies (RF/ID systems) have several advantages over comparable optical systems, such as better penetration of obstructing materials (e.g., clothing, soot) and easier electronic manipulation of the identifying signals. Microwave frequencies in particular are attractive due to relatively low radio noise and interference levels, wide available bandwidth for high-speed data transfer, and physically small high-gain antennas.
In a typical system, transponders (ranging from electronic ID badges to antitheft tags) are read, or interrogated, by a microwave beam which causes them to emit a coded response. Various types of response signals are in use, including simple back scatter with modulation in amplitude, ("An Automatic Vehicle ID System for Toll Collecting," Lawrence Livermore National Laboratory, Report No. UCRL-TB-113409, April 1993), phase (P. de Bruyne and P. Leuthold, "Radar Surveillance of Autobahn Toll), or both (i.e., SSB) (T. Ohta, H. Nakano, and M. Tokuda, "Compact microwave remote recognition system with newly-developed SSB modulation," IEEE MTT-S Digest, pp.957-960, 1990), or generation of a continuous or modulated harmonic of the interrogation signal (R. Page, "A Low Power RF ID Transponder," RF Design, pp. 31-35, July 1993).
In the prior art systems where an active response is generated, the responding signal was usually close to the frequency of the interrogating signal or was harmonically related thereto and therefor lay within the interference band of the interrogating signal. Also the transponder card needs a microwave source. As a result, false detection resulting from an interrogator receiving reflections of its own transmitted harmonics could arise.
What is needed is a transponder in the form of an ID card which is interrogated at frequency fi and generates a response frequency modulated with an identification code at microwave frequencies apart from the interrogation signal, away front interference in the interrogation band, and without the need for a microwave source on the transponder card. The frequencies should not be harmonically related to the interrogation signal in order to avoid the problem of false detection resulting from an interrogator receiving reflections of its own transmitted harmonics.